Step-Sequencing Basics

Fig. 1. The
granddaddy of
the modern step
sequencer is
the Serge TKB.
Introduced in
1980 and still
being built by
Sound Transform
Systems,
the TKB has
pressure-sensing
touchpads and
other advanced
features.

Mechanical devices that can play hands-free
music have a long history—the player piano,
for instance. When analog synthesizers took
the music world by storm in the 1970s, the tool
of choice for automating patterns of notes was
the step sequencer.

Compared to MIDI sequencing, step
sequencers may seem primitive. But today’s
models can do some surprisingly sophisticated
tricks. In this article we’ll introduce the concepts
behind step sequencing and look at a few of
the modern variations.

Some digital music systems, such as
Propellerhead Reason, include step sequencers.
Without getting into the pros and cons of
software emulations, we’ll concentrate on
the real thing—hardware step sequencing as
found in a modular analog synthesizer.

Baby Steps The frequency of an analog
oscillator is increased or decreased by
changing the level of the control voltage being
sent to it. In most systems, increasing the
voltage by one volt boosts the oscillator’s
pitch by an octave.

Fig. 2. The two
Pressure Points
modules in the lower
row (built by Make
Noise), together with
the narrow Brains
module to their left,
form an eight-step
sequencer with
touchpads. Above
the Pressure Points
is a Trigger Riot from
Tiptop Audio, a digital
module that produces
rhythmic trigger/gate
signals for analog
sequencing.

On a step sequencer, you’ll find one or
more rows of knobs (see Figure 1), usually
with eight or 16 knobs per row. For each row,
there’s a voltage output. In use, the sequencer
steps along the row or along the columns of
knobs, one knob or column at a time. At each
step, the voltage level set by a corresponding
knob is sent to that row’s output. The output
can be patched to the CV input of an oscillator.
Each time the sequencer selects a new knob,
the frequency of the oscillator changes,
producing a new musical pitch.

When the sequencer reaches the last
knob in the row, it hops back to the other
end, and the pattern of pitches repeats. If
there are three rows, the three knobs in each
column can be adjusted so that the oscillators
receiving their voltages will play the notes of
a chord. The voltages can be used for other
purposes too, such as changing the filter cutoff
frequency or the attack of a voltage-controlled
envelope generator.

An incoming signal tells the sequencer
when to step forward to the next knob.
This signal is called a clock, trigger, or gate.
Typically it’s a sharply rising voltage, such
as the leading edge of a square wave coming
from an LFO (low-frequency oscillator). The
sequencer senses the edge of the square wave
and responds by moving to a new step.

Generally, the clock signal is also used to
gate one or two envelope generators. These
will control the filter cutoff frequency and/or the
level of a VCA (voltage-controlled amplifier),
thus giving a musical shape to the sequencer’s
series of pitches.

That description would fit the sequencers
of the 1970s, as used by groups like Tangerine
Dream to produce hypnotic soundscapes. But
an endlessly looping set of eight or 16 notes
can get boring pretty quickly. So let’s spice
things up.

Making it Musical Rather than let the
LFO that is providing the clock signal cycle
at one tempo, use one of the sequencer’s row
outputs to control the frequency of the LFO.
The pattern of notes will still repeat endlessly,
but now some of them can be longer or shorter
than others, creating a rhythm. For irregular,
non-repeating rhythms, the LFO frequency
can be controlled by a different voltage source,
such as another LFO.

Some sequencers have a CV input for up/
down direction. When this input is receiving a
high voltage, the sequencer will step backward
rather than forward. By sending this input a
signal from a different LFO, we can get a “two
steps forward, one step back” pattern. This is
more interesting than a static pattern, and the
pattern will continue for more than eight or 16
steps before repeating.

Another CV input, when it receives a high
voltage, will cause the sequencer to reset to
step 1 on its next step. This can shorten the
pattern sometimes, but not always, again
producing a more interesting and varied
phrase. If the sequencer has a touchpad for
each step, it may be able to reset to the most-recently
touched step rather than always
jumping back to step 1. Touching and holding a
pad will cause the sequencer to stop advancing
and hold that step until you lift your finger.

When a sequencer with touchpads
isn’t receiving a clock signal, it becomes a
rudimentary keyboard. Each time you touch
a pad, the voltages from the knobs directly
above that pad will be the ones sent from the
row outputs. Because voltages are general-purpose
control signals, the knobs can be used
for whatever you like—changing the rate of an
LFO or the speed of an envelope generator’s
attack or decay, for example. Such keyboards
aren’t velocity-sensitive, but even so, they can
be used for expressive performances.

On some sequencers, each column of
knobs is associated with its own gate output.
When the sequencer is on a given step,
it sends a high voltage to that step’s gate
output. This signal can trigger an envelope
generator, so some of the steps can sound
different from others. The envelope might
open up a subsidiary VCA, for example,
sending a modulation signal to the oscillator
to change its tone.

Getting an analog sequencer to play in tune
is a challenge, because the knobs can make fine
adjustments: They’re not stepped. A module
called a quantizer makes it easier to produce
standard musical scales. This receives the
output of a sequencer’s knob row and adjusts
it up or down to the nearest 1/12 volt. With
oscillators built to the one-volt-per-octave
standard, the quantizer will ensure that the
sequencer’s output is in tune.

Giant Steps The future of analog modular
synthesis looks good. Many small companies
are building visionary new modules, including
a few that interface with a computer in novel
ways. Though step sequencers were once
considered an obsolete technology, today it’s
clear they’re here to stay.

Jim Aikin has written hundreds of product
reviews and tutorials for Electronic
Musician and other magazines over the
course of more than 30 years. His books
on music technology include Power
Tools for Synthesizer Programming (Hal
Leonard Publishing) and Csound Power!
(Cengage Learning).